I cannot connect my Mac mini to iCloud. I sign-in but "Fails to authenticate." I can sign in and have communication between my Mac Air, iPad and iPhone xs on iCloud. Just my Mac Mini is in isolation.

Bev2022 Said:

"Thanks. Tried restarting router. No success.: I talked with Apple Support I am still running Mojave. Apple tech said, after cycling through the loop of: Enter ID, then password, twirling ball,-thinking, then revert to enter ID. The suggested solution was to upgrade to current OS. There were some options to add another administrator or open SAFE mode to isolate issue but those are too complex for me."

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You are welcome.

Call Apple Support:

If this is too technical, then contact Apple Support. Perform a screensharing session with them, so they can eerily run you through the fix.

Apple’s Contact Information:

• Contact Us - Choose Locations
• Proceed from there as necessary
• Note: Phone calls are taking a bit to go through at the moment, due do lots of calls. And that makes sense, because with the Coronavirus Pandemic, many are at home at the moment, with much time on their hands to contact Apple. So, just stay on the line, and you will get thorough :)

Screenshare with the Apple Rep

To easily show the Apple Rep what is going on here, request a prompt for a screensharing session with the Apple Rep. That will allow them to see this firsthand, and upfront. Show this user through screensharing your Mac. Show them any paperwork or hover over something, through screensharing your iPhone.

Learn More at my User Tips: 

• iPhone: Screenshare your iPhone with the Camera App Enabled to Easily Converse Over-the-Phone with Apple Support - User Tip
• Mac: Screensharing MacOS FaceTime to Easily Converse Over-the-Phone with Apple Support - User Tip

Bev2022 Said:

"I cannot connect my Mac mini to iCloud. I sign-in but "Fails to authenticate." I can sign in and have communication between my Mac Air, iPad and iPhone xs on iCloud. Just my Mac Mini is in isolation."

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Refer to the link provided atop this post.

Try restarting your router.

Try logging in to iCloud.com from Safari.